Australian Antarctic Division: Leading Australia’s Antarctic Program

Sky and the seasons

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This week at Davis: 22 March 2013

As the meteorology tech guy, the first thing I'll be talking about is the weather. This Wednesday (20th March) is equinox. This means that the days and nights are of equal length, and the days are fast becoming shorter. On equinox the sun will rise in due east and set in due west, and from then until the next equinox in September the sun will be in the northern half of the sky, skimming closer and closer to the horizon until it doesn't rise at all (in June).

We are definitely feeling the chill. The weather through the summer seemed to hover around freezing, but now it is common to see temperatures of minus ten to fifteen degrees Celsius. On the second of March we had grease ice forming on the bay. At first I thought it would be blown out by the next wind, but this was two weeks ago and we have had pretty light winds since then. So now the ice is getting thicker and thicker, and it's easily enough to support the weight of an elephant seal. They have a little gap in the broken pancake ice by the beach but it will be interesting to see how much longer they can keep it open for.

Another interesting thing is the optical phenomena we have seen. Now the sun is setting, and lower in the sky, the ground is beginning to cool, and as a result the surface temperatures can be several degrees colder than the air a few hundred metres up. This is known as a temperature inversion. Funnily enough it tends to happen on the clear sunny days, as cloud can act as a blanket and give warmer temperatures. On a few of these days the temperature differentials have been so strong that the light actually bends slightly, towards the colder denser air on the surface. This results in a superior mirage, meaning the image you see is above where it really is.

On the other hand, when the sun sets over the ocean the light often passes over a warmer layer of air over the ocean surface. The sea water maintains a temperature of minus one point eight Celsius, so that is actually often warmer than the cold air coming from the ice plateau. As a result air is bent upwards, and this can be a good thing when you are trying to see a green flash. A green flash happens for only a few seconds, just after sunset. It is the last of the sun’s rays refracting around Earth’s surface, just like in a prism. I've only ever seen it once before, on the ship out of Hobart, but this one on 14 March went for probably three to five seconds and about half of a dozen of us saw it. I managed to get a photo but really, you had to be there!

Finally something to look out for at sunrise or sunset is the solar pillar. This is a vertical beam of light above the sun, caused by reflections off ice crystals in the atmosphere. We've all seen it a few times, and it can often be seen for up to half an hour or so. It is always good to have a camera close at hand.